The process and apparatus are used for aeration in gas/liquid reactions, preferably for aerating aerobic microbiological processes such as fermentation processes, in particular processes for the production of single-cell proteins.
It is known that, in the case of aerobic fermentation, the growth rate of the microorganisms, or rather their productivity, increases in relation to the degree of aeration. Processes and apparatus for commercial-scale fermentation are known from OS No. 24 36 793 and OS No. 25 54 440. In such processes, a culture solution is recycled in a system which consists of a vertical, partly parallel cycle with two reaction zones, namely rising zone and descending zone. In the rising zone the liquid flows upwards and is mixed with air bubbles. When all or part of the air bubbles on the surface of the liquid have disappeared, the remaining quantity of liquid flows down to the bottom of the descending zone, where it is mixed with air and flows up the rising zone. The air-lift pump action comes into effect.
The air can be admixed to the culture solution either by means of ejectors or by means of rotating aeration pumps as taught in OS No. 23 59 830. When these pumps are used, the culture solution is greatly accelerated, air is drawn in from the atmosphere due to the vacuum thus produced and mixed with the liquid. By appropriately controlling the liquid/gas mixture in the fermentation reactor, a cycle is maintained in which the mixture reaches the surface regularly, where the air on the surface of the liquid is more or less deaerated. Some of the known processes and apparatus also make use of the air-lift pump principle.
The circulation of liquid by means of a pump which handles the entire liquid quantity and which draws in air through holes in the pump wall, produces a high circulation velocity. Reducing the speed of the pump rotor will reduce both the circulation rate of the liquid and the intake of air through the holes in the pump. Thus, control is only possible within a narrow range.
The processes and apparatus which are known at present show characteristics which involve disadvantages for a number of reactions. Thus, a high liquid velocity in the ejector or in the rotary aeration pump is required to produce a vacuum which is sufficient to draw in atmospheric air. Consequently, a very high amount of liquid is circulated per unit of time, thus shortening the remaining reaction period in the rising zone of the fermentation vessel. Moreover, the high outlet velocity causes a heavy physical strain on the microorganisms and damages or destroys some of them. This results in a low conversion rate and possibly even in the undesired production of impurities. In addition, the high outlet velocity from conventional equipment produces a large number of small air bubbles which, although they provide a large material transfer area, have an insufficient rising velocity and thus an excessive residence period in the reaction liquid. In addition, the air-lift pump effect is limited and the separation of the tiny air bubbles on the surface of the liquid is inadequate. The descending liquid stream in the fermenter still contains too much gas per unit of volume. The circulation of gas-laden liquid by means of pumps requires more or bigger equipment.
Another apparatus is described in DE PS No. 1 667 042 which permits a narrow, easily adjustable bubble spectrum to be obtained. This device enables a uniform supply of air to be distributed across the bottom of the reactor with a relatively low, directly pumped quantity of liquid. This produces a uniform rise of bubbles across the reactor. However, at a high gassification rate and with liquids or reaction mixtures that have a tendency to foam, a closed foam layer is formed. Flotation of immiscible particles also occurs. A defined mixing current, i.e., a desired mixing quality cannot be achieved, particularly in the case of large fermenters.
The problem therefore emerged of eliminating the shortcomings of the processes and apparatus which are known at present.